Programming guidelines

From HaskellWiki

Programming guidelines shall help to make the code of a project better
readable and maintainable by the varying number of contributors.

It takes some programming experience to develop something like a
personal "coding style" and guidelines only serve as rough shape for
code. Guidelines should be followed by all members working on the
project even if they prefer (or are already used to) different
guidelines.

Surely same style choices are a bit arbitrary (or "religious") and
too restrictive with respect to language extensions. Nevertheless I hope
to keep up these guidelines (at least as a basis) for our project
in order to avoid maintaining diverging guidelines. Of course I want
to supply - partly tool-dependent - reasons for certain decisions and
also show alternatives by possibly bad examples. At the time of
writing I use ghc-6.4.1, haddock-0.7 and (GNU-) emacs with the latest
haskell mode.

What now follows are descriptions of program documentation, file
format, naming conventions and good programming practice (adapted form
Matt's C/C++ Programming Guidelines and the Linux kernel coding
style).

1 Documentation

Comments are to be written in application terms (i.e. user's point of
view). Don't use technical terms - that's what the code is for!

Comments should be written using correct spelling and grammar in complete
sentences with punctation (in English only).

"Generally, you want your comments to tell WHAT your code does, not HOW.
Also, try to avoid putting comments inside a function body: if the
function is so complex that you need to separately comment parts of it,
you should probably" (... decompose it)

Put a haddock comment on top of every exported function and data type!
Make sure haddock accepts these comments.

The \$Header\$ entry is automatically expanded by cvs (and will wrap
around). All other lines should not be longer than 80 (preferably 75)
characters to avoid wrapped lines (for casual readers)!

Expand all your tabs to spaces to avoid the danger of wrongly expanding
them (or a different display of tabs versus eight spaces). Possibly put
something like the following in your ~/.emacs file.

(custom-set-variables '(indent-tabs-mode nil))

The last character in your file should be a newline! Under solaris
you'll get a warning if this is not the case and sometimes last lines
without newlines are ignored (i.e. "#endif" without newline). Emacs
usually asks for a final newline.

The whole module should not be too long (about 400 lines)

3 Naming Conventions

In Haskell types start with capital and functions with lowercase
letters, so only avoid infix identifiers! Defining symbolic infix
identifiers should be left to library writers only.

(The infix identifier "\\" at the end of a line causes cpp preprocessor
problems.)

Names (especially global ones) should be descriptive and if you need
long names write them as mixed case words (aka camelCase). (but "tmp"
is to be preferred over "thisVariableIsATemporaryCounter")

Also in the standard libraries, function names with multiple words are
written using the camelCase convention. Similarly, type, typeclass and
constructor names are written using the StudlyCaps convention.

Some parts of our code use underlimes (without unnecessary uppercase
letters) for long identifiers to better reflect names given with
hyphens in the requirement documentation. Also such names should be
transliterated to camlCase identifiers possibly adding a (consistent)
suffix or prefix to avoid conflicts with keywords. However, instead of
a recurring prefix or suffix you may consider to use qualified imports
and names.

4 Good Programming Practice

"Functions should be short and sweet, and do just one thing. They should
fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
as we all know), and do one thing and do that well."

Most haskell functions should be at most a few lines, only case
expression over large data types (that should be avoided, too) may need
corresponding space.

The code should be succinct (though not obfuscated), readable and easy to
maintain (after unforeseeable changes). Don't exploit exotic language
features without good reason.

It's not fixed how deep you indent (4 or 8 chars). You can break the
line after "do", "let", "where", and "case .. of". Make sure that
renamings don't destroy your layout. (If you get to far to the right,
the code is unreadable anyway and needs to be decomposed.)

Bad:

case foo of Foo -> "Foo"
Bar -> "Bar"

Good:

case <longer expression> of
Foo -> "Foo"
Bar -> "Bar"

Avoid the notation with braces and semicolons since the layout rule
forces you to properly align your alternatives.

Respect compiler warnings. Supply type signatures, avoid shadowing and
unused variables. Particularly avoid non-exhaustive and
overlapping patterns. Missing unreachable cases can be filled in using
"error" with a fixed string "<ModuleName>.<function>" to indicate the
error position (in case the impossible should happen). Don't invest
time to "show" the offending value, only do this temporarily when
debugging the code.

For partial functions document their preconditions (if not obvious)
and make sure that partial functions are only called when
preconditions are obviously fulfilled (i.e. by a case statement or a
previous test). Particularly the call of "head" should be used with
care or (even better) be made obsolete by a case statement.

Usually a case statement (and the import of isJust and fromJust from
Data.Maybe) can be avoided by using the "maybe" function:

maybe (error "<ModuleName>.<function>") id $ Map.lookup key map

Generally we require you to be more explicit about failure
cases. Surely a missing (or an irrefutable) pattern
would precisely report the position of a runtime error, but these are
not so obvious when reading the code.

Do avoid mixing "let" and "where". (I prefer "let" and have auxiliary
function on the top-level that are not exported.) Export lists also
support the detection of unused functions.

If you notice that you're doing the same task again, try to generalize
it in order to avoid duplicate code. It is frustrating to change the
same error in several places.

4.2 Application notation

Many parentheses can be eliminated using the infix application operator "$"
with lowest priority. Try at least to avoid unnecessary parentheses in
standard infix expression.

f x : g x ++ h x

a == 1 && b == 1 || a == 0 && b == 0

Rather than putting a large final argument in parentheses (with a
distant closing one) consider using "$" instead.

Note that the final argument may even be an infix- or case expression:

map id $ c : l

filter (const True) . map id $ case l of ...

However, be aware that $-terms cannot be composed further in infix
expressions.

Probably wrong:

f $ x ++ g $ x

But the scope of an expression is also limited by the layout rule, so
it is usually save to use "$" on right hand sides.

Ok:

do y <- f $ l
++
do y <- g $ l

Of course "$" can not be used in types. GHC has also some primitive
functions involving the kind "#" that cannot be applied using "$".

Last warning: always leave spaces around "$" (and other mixfix
operators) since a clash with template haskell is possible.

(Also write "\ t" instead of "\t" in lambda expressions)

4.3 List Comprehensions

Use these only when "short and sweet". Prefer map, filter, and foldr!

Instead of:

[toUpper c | c <- s]

write:

map toUpper s

Consider:

[toUpper c | s <- strings, c <- s]

Here it takes some time for the reader to find out which value depends
on what other value and it is not so clear how many times the interim
values s and c are used. In contrast to that the following can't be clearer:

map toUpper (concat strings)

When using higher order functions you can switch easier to data
structures different from list. Compare:

map (1+) list

and:

Set.map (1+) set

4.4 Types

Prefer proper data types over type synonyms or tuples even if you have
to do more constructing and unpacking. This will make it easier to
supply class instances later on. Don't put class constraints on
a data type, constraints belong only to the functions that manipulate
the data.

Using type synonyms consistently is difficult over a longer time,
because this is not checked by the compiler. (The types shown by
the compiler may be unpredictable: i.e. FilePath, String or [Char])

Take care if your data type has many variants (unless it is an
enumeration type.) Don't repeat common parts in every variant since
this will cause code duplication.

Bad (to handle arguments in sync):

data Mode f p = Box f p | Diamond f p

Good (to handle arguments only once):

data BoxOrDiamond = Box | Diamond

data Mode f p = Mode BoxOrDiamond f p

Consider (bad):

data Tupel a b = Tupel a b | Undefined

versus (better):

data Tupel a b = Tupel a b

and using:

Maybe (Tupel a b)

(or another monad) whenever an undefined result needs to be propagated

4.5 Records

For (large) records avoid the use of the constructor directly and
remember that the order and number of fields may change.

Take care with (the rare case of) depend polymorphic fields:

data Fields a = VariantWithTwo
{ field1 :: a
, field2 :: a }

The type of a value v can not be changed by only setting field1:

v { field1 = f }

Better construct a new value:

VariantWithTwo { field1 = f } -- leaving field2 undefined

Or use a polymorphic element that is instantiated by updating:

empty = VariantWithTwo { field1 = [], field2 = [] }

empty { field1 = [f] }

Several variants with identical fields may avoid some code duplication
when selecting and updating, though possibly not in a few
depended polymorphic cases.

However, I doubt if the following is a really good alternative to the
above data Mode with data BoxOrDiamond.

4.9 Glasgow extensions and Classes

Stay away form extensions as long as possible. Also use classes with
care because soon the desire for overlapping instances (like for lists
and strings) may arise. Then you may want MPTC (multi-parameter type
classes), functional dependencies (FD), undecidable and possibly incoherent
instances and then you are "in the wild" (according to SPJ).

5 Final remarks

Despite guidelines, writing "correct code" (without formal proof
support yet) still remains the major challenge. As motivation to
follow these guidelines consider the points that are from the "C++
Coding Standard", where I replaced "C++" with "Haskell".

Good Points:

programmers can go into any code and figure out what's going on

new people can get up to speed quickly

people new to Haskell are spared the need to develop a personal style and defend it to the death

people new to Haskell are spared making the same mistakes over and over again

people make fewer mistakes in consistent environments

programmers have a common enemy :-)

Bad Points:

the standard is usually stupid because it was made by someone who doesn't understand Haskell